The present invention relates to an exhaust gas cleaning or purifying apparatus and method of effectively removing NOx in an exhaust gas while preventing or suppressing poisoning of a NOx reduction catalyst with SOx contained in the exhaust gas of an internal combustion engine such as an automobile engine.
In recent years, a lean-burn engine operated in a fuel-lean condition of an air-to-fuel ratio is regarded as a promising internal combustion engine for a vehicle from the viewpoints of reduction of fuel consuming quantity and prevention of global warming by carbon oxide gas. However, the exhaust gas of the engine becomes an oxidizing atmosphere in which O.sub.2 concentration of the exhaust gas exceeds the stoichiometric ratio necessary for complete combustion of reductive components contained in the exhaust gas (hereinafter, such an oxidizing atmosphere is referred to as an oxidizing atmosphere). A conventional three-way catalyst efficiently cleans NOx, HC and CO under a reducing atmosphere in which O.sub.2 concentration contained in the exhaust gas is below the stoichiometric ratio necessary for completely burning reductive components contained in the exhaust gas (hereinafter, such reductive atmosphere is referred to as a reductive atmosphere), and does not exhibit sufficient NOx purifying performance under the oxidizing atmosphere. Therefore, it is required to develop a catalyst capable of purifying NOx, HC and CO under a reductive atmosphere, and particularly, effectively purifying NOx.
As the apparatus of purifying an exhaust gas for a lean-burn engine, WO93/07363 and WO93/08383 propose that an NOx absorbent is arranged in an exhaust gas passage. The NOx absorbent absorbs NOx in the exhaust gas during lean fuel combustion and discharge NOx when the oxygen concentration in the exhaust gas decreases.
Since fuel contains a sulfur component, the exhaust gas contains SOx, mainly SO.sub.2. The SOx reacts with NOx absorbing components in a NOx absorbent to produce sulfuric salts or sulfurous salts. As a result, there arises the so-called SOx poisoning problem by which the NOx absorbing capability is decreased. Turning the NOx absorbent into sulfuric salt or sulfurous salts becomes intensive in the oxidizing atmosphere of exhaust gas produced by lean combustion. Therefore, improvement of the resistance to SOx poisoning is important to develop a catalyst coping with the lean-burn engine.
JP 2605553 specification discloses an exhaust gas purifying apparatus in which a sulphur capturing apparatus is arranged upstream of a NOx absorbent in an exhaust path of an internal combustion engine. The specification describes that sulphur, as well as NOx, is absorbed in the NOx absorbent, the absorbed sulphur is not released from the NOx absorbent and accumulated gradually in the NOx absorbent even if an air fuel ratio of an exhaust gas flowing in the NOx absorbent is turned to be fuel-rich, and the NOx absorbent comes almost not to absorb NOx as an amount of sulphur within the NOx absorbent increases. The invention of JP 2605553 is to suppress gradual accumulation of sulphur into the NOx absorbent by providing the sulphur capturing apparatus at an upstream side of the NOx absorbent.
JP 2605559 specification, JP 2605571 specification and JP 2605580 specification each disclose that a SOx absorbent is arranged in an exhaust path of an internal combustion engine at an upstream side of a NOx absorbent, which SOx absorbent absorbs SOx when an air fuel ratio of an inflow exhaust gas is in fuel lean side and releases the absorbed SOx when the air fuel ratio of inflow exhaust gas comes to a fuel rich side. Further, the specifications disclose that as the SOx absorbent, an absorbent, which has iron, manganese, nickel, tin, titanium, copper, lithium, etc. each supported on a support comprising alumina, is used, and that it is preferable to support platinum on the support of the SOx absorbent. Further, the JP 2605559 specification discloses an air fuel ratio controlling means provided for controlling an air fuel ratio of an exhaust gas flowed in the SOx absorbent to be rich when SOx is to be released from the SOx absorbent.
Japanese Patent Application Laid-Open No.8-192051 discloses suppressing SOx-poisoning of a NOx absorbent, using a support made of a complex oxide of titanium and zirconium.
Japanese Patent Application Laid-Open No.9-155191 discloses suppressing sulphur poisoning of a NOx absorbent by supporting yttrium and an NOx absorbing component on a support.
The method in which the SOx absorbent is provided upstream of the NOx absorbent is effective when the SOx absorbent has a very high SOx absorption capacity and is able to sufficiently absorb SOx in the exhaust gas. However, when the SOx absorption capacity is low and SOx in the exhaust gas can not be sufficiently absorbed, there is a problem that SOx not absorbed by the SOx absorbent accumulates in the NOx absorbent. Further, there is a problem that the SOx absorption capacity of the SOx absorbent decreases according to passage of time, the SOx not absorbed by the SOx absorbent according thereto is absorbed in the Nox absorbent. According to the disclosure of JP 2605553 specification, SOx absorbed in the NOx absorbent is not released from the NOx absorbent, and the NOx absorption capacity of the NOx absorbent gradually decreases by accumulation of SOx.